Cool drying is, as is known, based on the principle that by lowering the gas temperature the moisture in the gas condenses, after which the condensate is separated in a liquid separator and after which the gas is heated again such that this gas is no longer saturated.
It is known that in most cases compressed air, supplied by a compressor for example, is saturated with water vapour or, in other words, it has a relative humidity of 100%. This means that condensation occurs in the event of a temperature drop to below the ‘dew point’. Because of the condensed water, corrosion occurs in the pipes and tools that draw off compressed air from the compressor, and equipment can present premature wear.
It is consequently necessary to dry this compressed air, which can be done in the aforementioned way by cool drying. Air other than compressed air or other gases can also be dried in this way.
In the known method the cooling circuit is controlled via two separate controls: a control of the expansion valve on the one hand and a control of the hot gas bypass valve on the other.
The expansion valve is used to expand just enough coolant so that the coolant is always superheated when entering the compressor.
As a result of this superheating the liquid coolant present can be evaporated before it is guided to the compressor, in order to provide optimum protection of the compressor against liquid coolant.
The superheating of the coolant can be determined on the basis of the measurements of the evaporator pressure and the evaporator temperature, and it can be determined whether the expansion valve has to be opened more or less in order to control the superheating of the coolant in this way.
The hot gas bypass valve is used to ensure that the evaporator pressure is not too low so that the air in the heat exchanger is not cooled too greatly, as otherwise the condensate would freeze.
By tapping off a certain quantity of coolant in the form of hot gas from the cooling circuit and driving it through the bypass pipe across the compressor, the evaporator pressure, and automatically the coolant temperature, is kept under control. This can be necessary for example with a variable load of the cooling circuit.
In this way the cooling capacity of the device can decrease and prevent the condensate in the heat exchanger from freezing or the temperature of the coolant falling too greatly.
The hot gas bypass valve is hereby controlled by a control unit that is connected in a known way to one or more sensors.
These sensors are affixed for example to determine the evaporator pressure. When these sensors register an evaporator pressure that is too low, the control unit sends a signal to the hot gas bypass valve to open this last-mentioned. In this way a certain quantity of coolant is guided through a bypass pipe across the compressor so that the cooling capacity of the cooling circuit decreases.
A disadvantage is that two separate controls have to be provided, which makes the method complicated.
Moreover the control of the expansion valve influences the control of the hot gas bypass valve and vice versa.
Indeed, if the expansion valve opens or closes to obtain fixed superheating even with a variable load, the evaporator pressure will rise or fall. As a result the hot gas bypass valve must also be adjusted to be able to adjust the evaporator pressure to the desired setpoint. The desired evaporator pressure depends on the load.
In other words a change of the state of the hot gas bypass valve will ensure that the expansion valve also has to be adjusted.
This makes it difficult to guarantee the good operation of the cooling circuit.